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When a jet escapes from a collapsed star, it strikes a cocoon of stellar debris. Credit: Ore Gottlieb/CIERA/Northwestern University

So far, astrophysicists have only detected gravitational waves from binaries, the mergers of two black holes, two neutron stars, or one of each. While astrophysicists should theoretically be able to detect gravitational waves from a single non-binary source, they have yet to discover these elusive signals.

Now Northwestern University researchers suggest looking at a new, unexpected and wholly unexplored place: the turbulent, energetic cocoons of debris that surround dying massive stars.

For the first time ever, researchers have used state-of-the-art simulations to demonstrate that these cocoons can emit gravitational waves. And, unlike gamma-ray bursts, the cocoons’ gravitational waves should fall within the frequency band detectable by the Laser Interferometer Gravitational-Wave Observatory (LIGO).

“To date, LIGO has only detected gravitational waves from binary systems, but one day it will detect the first non-binary source of gravitational waves,” said Ore Gottlieb of Northwestern, who led the study. “Cocoons are one of the first places we should look for this type of source.”

Gottlieb will present this research during a virtual press conference at the 242nd meeting of the American Astronomical Society. “Launched and Turbulent Stellar Deaths: New Sources of Gravitational Waves Detectable by LIGO” will take place at 12:15 PM EDT on Monday, June 5, as part of a session on “Discoveries in Distant Galaxies.”

Gottlieb is a CIERA Fellow at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). The study’s Northwestern co-authors include professors Vicky Kalogera and Alexander Tchekovskoy, postdoctoral fellows Sharan Banagiri and Jonatan Jacquemin-Ide, and graduate student Nick Kaaz.


The evolution of the jet cocoon from the birth of the black hole to the detachment from the star (the color map is the logarithm of the amplitude of the off-axis strain and the sound reflects the GW frequency). Credit: Ore Gottlieb/CIERA/Northwestern University

The new source was ‘impossible to ignore’

To conduct the study, Gottlieb and his collaborators used new cutting-edge simulations to model the collapse of a massive star. When massive stars collapse into black holes, they can create powerful outflows (or jets) of particles that travel near the speed of light. Gottlieb’s simulations modeled this process from the moment the star collapses into a black hole until the jet escapes.

Initially, he wanted to see if the accretion disk that forms around a black hole could emit detectable gravitational waves. But something unexpected kept emerging from his data.

“When I calculated gravitational waves from the vicinity of the black hole, I found another source that was interrupting my calculations, the cocoon,” Gottlieb said. “I tried to ignore it. But I found it was impossible to ignore it. Then I realized that the cocoon was an interesting source of gravitational waves.”

When the jets collide in the collapsing layers of the dying star, a bubble, or “cocoon,” forms around the jet. Cocoons are turbulent places, where hot gases and debris mix randomly and expand in all directions from the jet. As the energy bubble accelerates from the jet, it perturbs spacetime to create a ripple of gravitational waves, Gottlieb explained.

“A jet takes off deep inside a star and then blasts its way out,” Gottlieb said. “It’s like when you drill a hole in a wall. The spinning drill bit hits the wall and the debris shoots out of the wall. The drill bit energizes that material. Likewise, the jet hits the star, causing the material to heat up of the star and come out.This debris forms the warm layers of a cocoon.


360-degree view of the dying star’s cocoon (color map is logarithmic strain amplitude). Credit: Ore Gottlieb/CIERA/Northwestern University

Call to action to watch cocoons

If the cocoons generate gravitational waves, then LIGO should be able to detect them on its next runs, Gottlieb said. Researchers have typically looked for single-source gravitational waves from gamma-ray bursts or supernovae, but astrophysicists doubt LIGO can detect them.

“Both jets and supernovae are very energetic explosions,” Gottlieb said. “But we can only detect gravitational waves from higher-frequency asymmetrical outbursts. Supernovae are quite spherical and symmetrical, so spherical outbursts do not change the balanced mass distribution in the star to emit gravitational waves. Gamma-ray bursts last tens of seconds , so the frequency is very small, lower than the frequency band LIGO is sensitive to.”

Instead, Gottlieb asks astrophysicists to redirect their attention to cocoons, which are both asymmetrical and highly energetic.

“Our study is a call to action for the community to consider cocoons as a source of gravitational waves,” he said. “We also know that cocoons emit electromagnetic radiation, so they could be multi-messenger events. By studying them, we could learn more about what happens in the innermost part of stars, the properties of jets and their prevalence in stellar explosions.”

The study is titled “Jet and turbulent stellar deaths: new sources of gravitational waves detectable by LVK”.

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